I do not believe that there is a specific age. I think that it varies depending on the maturity level of the child, and if that child would know what to do in case of emergency.
You would not want to leave a child of 6 at home alone but I would say at 11-13 you could for short lengths of time.
Again, this would depend on the maturity level and also if there are neighbors that are home, other children in a neighborhood that are there.
I began riding the bus home in middle school and it was just me and my sister a couple years older for over an hour until my stepmother came home. We did chores or homework but were not allowed to leave the house until she came home. I do not believe that my sister was allowed to stay home alone prior to that due to the schedule and school she went to at that time.